They do meet the independent third party standard, but that’s because the Zigbee 3.0 standard allows individual device manufacturers to use “manufacturer proprietary” clusters, which then mean the devices can be hard to use with other companies’ hubs. 
Matter will be different, though. Interoperability is one of its key tenets, so they are not providing the same option in the same way. Base functionality has to work the same for every Matter-certified device: on/off/dim. But manufacturers can still provide unique advanced features like child lock or adjustable ramp rates that will only work with their own platform.
That will be better for customers than Zigbee, where manufacturer-proprietary code support might be needed even to turn the device on. At least the endpoints. 
If you can wait a few months, Tuya is planning to add Matter support to a few of their hubs AND to make at least one of their hubs a “matter bridge.” That means it would work like HomeKit in or hue out: you would add the Tuya devices to their own hub and then add that hub to your SmartThings account via Matter. It wouldn’t support all devices or all features, but setup should be easy and no custom code required. So if you like the Tuya devices, that would be an easy way to use them. 
SMARTTHINGS AND MATTER: ONE WAY IN
BTW, SmartThings does NOT plan to make its hubs a “matter bridge,” so its matter integration will be one way in. You won’t be able to add a SmartThings/Aeotec hub with its attached devices to other Matter-certified apps. But you should have more devices to choose from to add to the ST app.
You can read more about Matter in the following thread:
Matter - smart home connectivity standard (formerly Project CHIP)
BACK TO COLOUR CHOICES
most manufacturers for the UK market solve the aesthetic choice issue by providing zwave modules that can be placed inside the wall for mains powered devices, or that have interchangeable cases for batterypowered devices, and then you can easily swap colours.
For mains powered options, the Aeotec nano zwave line works well with SmartThings and you can use any refractive (momentary) dumb face plate you like, so lots of decorating options.
But you didn’t want mainspowered.
And now we run into a SmartThings-specific issue. 
For years several zwave manufacturers in Europe have offered multi button battery powered actuators that have changeable faceplates.
As one community developer put it several years ago put it:
The problem is SmartThings doesn’t provide official feature support for the zwave command sets needed to make these work. So the community developer ended up having to write both a groovy DTH and a groovy smartapp, both of which are obsolete with the new architecture. And it’s unlikely there will ever be a replacement.
These devices work just fine with pretty much every other certified zwave hub out there, but SmartThings’ insistence on using its own unique overlay architecture makes them unavailable to SmartThings customers now, which is why I didn’t mention them before.
So Tuya isn’t the only company using manufacturer-proprietary code. Just sayin’…